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BEIRUT – Thousands of Syrians rushed toward the Turkish border Friday, fleeing a fierce government offensive and intense Russian airstrikes near Syria’s largest city of Aleppo.

Turkey, an ally of the Syrian opposition, promised humanitarian help for the displaced civilians, including food and shelter, but it did not say whether it would let them cross into the country, already burdened with hundreds of thousands of refugees.

The U.N. estimated that nearly 40,000 newly displaced people have massed in recent days in several border areas of northern Syria, including about 20,000 near the Bab al-Salam border crossing. Turkish authorities increased security at the crossing and the pro-government A Haber news channel said all police and military leaves were cancelled.

The international aid group Mercy Corps said that among those fleeing toward Turkey were residents of rebel-held areas of Aleppo who feared they would soon be besieged by government forces, while others were running from troops advancing in rural areas.

The Syrian government offensive began earlier this week in rural areas north of Aleppo, the provincial capital, and appears aimed at eventually encircling the city. Forces loyal to President Bashar Assad captured several towns and villages, driving a wedge into rebel-held areas and cutting off a supply road to Turkey.

Once Syria’s thriving commercial centre, Aleppo has been divided since 2012 between government- and rebel-controlled districts. A government siege of rebel strongholds could isolate tens of thousands of civilians and would deal a devastating blow to the morale of groups fighting to topple Assad for the last five years.

“It does look like a terrible humanitarian situation inside of Syria and it is poised to get worse. And that is something that we continue to be quite concerned about,” Earnest said.

“There is no denying that the efforts of the Russian military to buck up and strengthen the Assad regime’s grip on power only gives the Assad regime less of an incentive to come to the negotiating table and act constructively in conversations there,” he added.

U.S. Secretary of State John Kerry accused Russia of using imprecise “dumb bombs” that have killed large numbers of civilians.

“This has to stop. Nobody has any question about that,” Kerry told reporters at the State Department.

NATO chief Jens Stoltenberg said the “intense Russian airstrikes mainly targeting opposition groups in Syria are undermining the efforts to find a political solution to the conflict.”

A U.N.-led attempt to launch indirect talks between a government delegation and opposition representatives in Geneva was adjourned Wednesday amid acrimonious bickering. U.N. envoy Staffan de Mistura said the process will resume Feb. 25.

The opposition’s chief negotiator, Mohammed Alloush, told The Associated Press late Thursday that his delegation is unlikely to return to Geneva because of what he said was a “merciless” bombing campaign by Russia and the Syrian air force this week.

At the United Nations, Russian Ambassador Vitaly Churkin said Moscow plans to present new ideas on how to restart the talks, including a cease-fire, at a Feb. 11 meeting of key countries in Munich. He said Moscow hopes others in the 17-nation group will “shoulder responsibility” in restarting the talks.

Tensions ran high outside the U.N. Security Council, as Britain and France blamed Syria’s offensive near Aleppo for the suspension of talks.

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French Ambassador Francois Delattre said the opposition couldn’t be expected to negotiate “with a gun to their heads,” and British Ambassador Matthew Rycroft said Churkin “needs to look in the mirror and understand where the responsibility lies.”

The Syrian rebels were able to hold positions in the Aleppo area before, but the Russian bombing, along with reinforcements sent to Assad by his allies in Iran and the Lebanese Hezbollah militia, appear to have tilted the balance in the battlefield.

London-based analyst Ayman Kamel, head of the Middle East and North Africa program at the Euroasia Group, said he expects Assad’s forces to regain control of Aleppo at some point this year, barring further foreign intervention.

“I don’t think Aleppo is necessarily the most important city in the country, but it is very symbolic,” he said. “If rebels lose their presence in Aleppo, then they are largely going to be demoralized, and it’s part of a larger campaign of them losing influence in northern Syria.”

Russia began its airstrikes in late September, ostensibly going after Islamic State extremists who control large areas of northeastern Syria. However, critics say Russian warplanes have struck a wide range of opposition targets in order to bolster Assad, a longtime ally.

Russia’s Defence Ministry said its warplanes hit 875 targets across Syria this week, including in the area of the current offensive.

Mercy Corps, which has been delivering food to civilians in northern Syria, said it had to stop distributions in opposition-held areas of Aleppo earlier this week because the sole access road became too dangerous.

The Russian airstrikes north of Aleppo have “hugely increased” in the past two weeks, said Rae McGrath, head of Mercy Corps operations in Turkey and northern Syria.

The human rights advocacy group Amnesty International urged Turkey to admit the displaced. Turkey “must not close its doors to people in desperate need of safety,” said Amnesty’s Global Issues Director Sherif Elsayed-Ali.

Turkish TV showed Syrians walking between long rows of large white tents at Bab al-Salam, and Davutoglu said tens of thousands more were on the move.

Abdulgani Fettah told Turkey’s state-run Anadolu Agency that he escaped to the border after his hometown of Bab came under heavy bombardment.

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“We are asking that Turkey looks after us and opens its doors to us,” Fettah said. “We are in difficulty because of the cold. There are sick people, children, women, and wounded people. They came to the border in difficult conditions.”

There were conflicting estimates of the number of displaced near the border.

The U.N. said up to 20,000 people gathered at the Bab al-Salam crossing. Farhan Haq, the deputy spokesman for the U.N. secretary-general, said another 5,000 to 10,000 people reached the town of Azaz and that 10,000 arrived in the town of Afrin.

The Turkish Islamic charity IHH said about 50,000 people arrived since Thursday in the area of Bab al-Salam. The group is erecting tents on the Syrian side of the border, said spokesman Serkan Nergis. The charity runs about 10 camps for displaced Syrians on the frontier.